Ceramic paper generally refers to high temperature resistant, insulating inorganic sheet material having a thickness of up to about 1/4 of an inch and predominantly comprising ceramic fibers. Such materials are referred to as "paper" because of their resemblance to wood pulp papers and because they can be produced on conventional papermaking machines. Such paper is used for a wide variety of industrial applications. More recently, the automotive industry has become interested in ceramic paper for use in the inflator filter units of automotive airbags.
For several years, automotive airbag manufacturers have sought ways to reduce costs by making more efficient use of the very expensive propellant or generant contained therein. One way that automotive airbag manufacturers have attempted to do this is to design inflators which burn hotter. It has been found that the hotter the generant systems within the inflators burn, the more optimum the completion of the reaction which, in turn, allows the airbag manufacturer to use less propellant or generant in the inflator, thereby reducing cost.
For many years, azide generant systems which include an azide component, such as sodium azide, have been utilized. In conjunction therewith, compatible filtration media was developed which typically centered on the use of fiberglass and/or ceramic fiber papers. Such papers typically comprised refractory ceramic fibers bound together by a latex binder system. However, often the latex binder was "burned out" so as to minimize additional organic components. These ceramic papers served three main functions in the typical inflator containing azide generants. First, the papers had to filter out the unwanted solid and liquid by-products left over from the combustion reaction of the azide generants. This meant that the papers had to have the proper mechanical means for entrapping the solid metal waste and the proper chemical means for entrapping the liquid metal oxide waste. Second, the papers had to control the flow rate of the nitrogen gas as it exited the inflator so as to fill the airbag properly. In other words, the papers had to have the proper porosity. Third, the papers served as insulators. The nitrogen gas liberated from the combustion reaction with the inflator needed to be cooled from approximately 2000.degree. F. (1093.degree. C.) to about 200.degree. F. (93.degree. C.). These fibers, in conjunction with the wire mesh which served as a heat sink, enabled the paper to serve as a barrier to heat. Thus, these prior art papers perform effectively when azide generants are utilized.
Recently, however, non-azide generants have been introduced to the automotive airbag industry. The use of these non-azide generants in airbag inflators is seen as a technological improvement over prior azide generants since many of these new non-azide generant systems burn much hotter and at higher pressures than their azide predecessors. Furthermore, these non-azide generants, e.g., guanidine, are not mutagenic and have a low toxicity as compared to the sodium azide generant which is mutagenic and toxic.
As a result of this advancement in the use of non-azide generants in airbag inflators, the filter media requirements for these new generant systems have changed significantly as well. That is, current refractory ceramic filter media used with azide generant inflators do not work as well with the non-azide technology because the non-azide inflators generate less reactive and smaller particle size by-products than their azide predecessors and produce chemically different by-products, including gases. Moreover, the filter media tends to load up with particulate and rupture under the higher temperature and pressure conditions associated with their use. In other words, while the refractory ceramic papers which act as filter media for azide generant systems continue to perform the latter two functions adequately (i.e., to control the flow rate of the nitrogen gas and to act as an insulator) for non-azide generants systems, they do not as effectively filter out the resultant by-products of the combustion reaction. Thus, conventional refractory ceramic filter papers do not work as well in these non-azide applications.
Accordingly, the need exists for a paper composite suitable for use as a hot gas filter media with non-azide, as well as azide, generants.
Heretofore, the use of carbon fibers in filtration paper technology and, more particularly, airbag inflator filter technology has, upon information and belief, been extremely limited. It is believed that carbon fiber paper containing carbon polyacrylonitrile (PAN) and carbon pitch fibers has heretofore been used as one ply of a two-ply sandwich wherein refractory ceramic fibers comprised the ceramic paper used in the other ply. However, these carbon fibers are not surface activated, have relatively low surface area, and have high tensile properties associated with rigidity and strength, not flexibility and filtering. Moreover, it is possible that such carbon fibers could give off other undesirable by-products. No single-ply paper composite containing both activated carbon fibers and refractory ceramic fibers is believed known, particularly for use as the filter in the inflator unit of an automotive airbag.